Homologous recombination is critical for repairing double strand breaks (DSBs), but it likely evolved primarily as a mechanism for repairing broken replication forks. DNA replication forks can break down when they encounter DNA lesions.

In order to detect homologous recombination in adult animals, we genetically engineered a DNA substrate wherein recombination between two non-functional copies of enhanced yellow fluorescent protein (EYFP) gives rise to fluorescent cells.

Rare recombinant cells can be seen in the pancreatic tissue of the fluorescent yellow direct repeat (FYDR).

A limitation of FYDR mice is that many tissues do not express the construct, so fluorescence cannot be detected. We therefore targeted a recombination reporter to the Rosa26 locus, which is known to be highly expressed.

Recombinant cells can be detected in almost all tissues of the Rosa26 direct repeat (RADR) mice.